The present invention relates to a sensor for the detection of the movement of an encoder to be monitored with at least two sensor elements (6, 7), which are arranged in a way offset relative to each other in the direction of movement of the encoder, and with a processing circuit (10, 14, 15), which converts the element output signals (S1, S2) at the outlets of the sensor element (6, 7) to a sensor output signal (S5) that describes the movement of the encoder. the processing circuit (10, 14, 15) includes a test circuit (14), which monitors the element output signals (S1, S2) at least indirectly and disables or corrects the sensor output signal (S5) when errors of the element output signals (6, 7) are detected. The invention also relates to a method for monitoring and eliminating sensor faults in a sensor (3) for the detection of the movement of an encoder with at least two sensor elements (6, 7), which are arranged in a way offset relative to each other in the direction of movement of the encoder, and with a processing circuit (10, 14, 15), which converts the element output signals (S1, S2) at the outlets of the sensor elements (6, 7) to a sensor output signal (S5) that describes the movement of the encoder. A test circuit (14) in a processing circuit (10, 14, 15) monitors the element output signals (S1, S2) at least indirectly and disables or corrects the sensor output signal (S5) when errors of the element output signals (6, 7) are detected.
Devices for the detection of rotational speeds in motor vehicles are principally known in the art. Basically, they comprise an encoder and a sensor magnetically coupled to the encoder via an air slot. The encoder is a machine element, which is mechanically connected to the rotating ring of a wheel bearing and carries an incremental angular scale. Said angular scale is designed as an integral sequence of magnetically alternating, differently effective areas forming a circular encoder track. It is conventional practice to use toothed wheels, ferromagnetic perforated discs or permanent-magnetized structures, e.g. magnetized wheel bearing seals as encoders. The sensor reacts to the periodic changes between tooth/gap or hole/web or north/south pole with a periodic electric signal reproducing the incremental angular spacing as a temporal voltage or current variation. Induction coils, magneto-resistive bridges and Hall elements are used as sensorially active components, being partly operated in combination with additional electronic circuits. It is usual to designate sensors as ‘active sensors’ when they require a current supply for operation and as ‘passive’ sensors when they do not need an additional current supply for operation, exactly as induction coils.
To be able to realize larger air gap lengths in sensor assemblies of this type, proposals have already been made to bisect the incremental bearing discrimination of the encoder and to compensate it again thereafter by doubling mechanisms with the use of sensors being locally shifted in relation to each other. Thus, DE 199 06 937 recommends using two Giant Magnetoresistive Effect sensors whose local positioning in relation to each other brings about a phase shift of roughly 90°. The signals (S1, S2) of the two sensors are amplified, led through threshold value switches and exclusively OR-operated. Another objective is to determine the direction of rotation by means of flip-flop circuits. Further proposals are directed to arranging the sensors jointly on one chip in order to be able to maintain the distance between both sensors as precisely as possible.
The application of this prior art is obstructed in several ways in practical operations. Thus, it is necessary to combine identical sensors with encoders of different modules (module=reading diameter/encoder number of cycles) for the case of application of the detection of the rotational speed in an automobile. According to experience, the module range is between 1.2 mm and 2.5 mm, that means, a ratio of the possible modules of 2.5/1.2=roughly 2 must be covered. In order to preserve always a phase shift of roughly 90°, it would be necessary according to this provision to keep on stock a large number of different sensors adapted to different modules. This necessity counteracts the goal of an economical manufacture and high quality of large piece numbers of an equal product. When module adaptation is omitted, another shortcoming that occurs involves that, with increasing phase deviations from the nominal value of 90°, each of the exclusively OR-operated sensorial channels contributes an individually fluctuating pulse-duty factor to the total signal, with the result of inadmissibly increasing the jitter for the operation of modern brake controllers.
When the two phase-shifted signals S1 and S2 of the sensor elements are processed appropriately and are then united for doubling the output signal, as has been described hereinabove, the nominal frequency f2 is achieved.
The sensors known from prior art inhere the problem that errors occur in the output signals of the sensor elements. These errors change the output signal of a sensor element in such a fashion that it deviates from its typical course, which corresponds to the application, or that the signal variation suffers from disturbances. These errors, which must be eliminated, can be due to a variety of different effects. In this connection, the effect is referred to as an example that the sensor signal is disturbed or corrupted due to an inexact distance or an inexact positioning between encoder and sensor or due to variations in distance. Among others, these disturbances frequently represent harmonic wave components in the sensor signal or different frequency fluctuations of the signal. These errors can become conspicuous in the course of further signal processing. In some cases, the result is even that the errors are amplified, in particular when the respective signal amplitude is evaluated by way of defined threshold values, or, respectively, the further signal variation orients itself by threshold values of this type. In this regard, particularly amplified frequency instabilities can occur in addition to intensified amplitude errors, because disturbances, which are revealed in spurious harmonic wave components and whose amplitude value is in the range of a threshold value imply quick abrupt changes in amplitude.
Thus, the invention is based on a sensor for the detection of the movement of an encoder to be monitored with at least two sensor elements (6, 7), which are arranged in a way offset relative to each other in the direction of movement of the encoder, and with a processing circuit (10, 14, 15), which converts the element output signals (S1, S2) at the outlets of the sensor element (6, 7) to a sensor output signal (S5) that describes the movement of the encoder. the processing circuit (10, 14, 15) includes a test circuit (14), which monitors the element output signals (S1, S2) at least indirectly and disables or corrects the sensor output signal (S5) when errors of the element output signals (6, 7) are detected. Additionally, the invention is based on a method for monitoring and eliminating sensor faults in a sensor (3) for the detection of the movement of an encoder with at least two sensor elements (6, 7), which are arranged in a way offset relative to each other in the direction of movement of the encoder, and with a processing circuit (10, 14, 15), which converts the element output signals (S1, S2) at the outlets of the sensor elements (6, 7) to a sensor output signal (S5) that describes the movement of the encoder. A test circuit (14) in a processing circuit (10, 14, 15) monitors the element output signals (S1, S2) at least indirectly and disables or corrects the sensor output signal (S5) when errors of the element output signals (6, 7) are detected.